1. Field of the Invention
This invention relates to a method of making a cellular body from a high borosilicate composition, and more particularly, to a method of cellulating high borosilicate compositions by forming an intimate mixture of the pulverulent constituents and a portion of previously cellulated high borosilicate compositions and thereafter subjecting the mixture to cellulating temperatures to form a cellular body.
2. Description of the Prior Art
Conventional glasses that are melted in conventional glass melting tanks contain about 70% by weight silica. High silica glasses that are melted in special high temperature melting tanks contain about 80% by weight silica. It is extremely difficult to melt glasses containing above 80% by weight silica or above 90% by weight of a combination of silica and alumina in either conventional glass melting tanks or in the special high temperature glass melting tanks.
The process of melting the constituents in a glass tank consists of decomposing all or some of the constituents, forming a liquid mixture of the constituents, removing the trapped gases and improving the homogeneity of the molten mass. The process of removing gases and improving the mixing and homogeneity depends on a number of parameters especially the viscosity of the molten mass. The melting process requires liquidity and a reduction in the viscosity of the molten mass and usually takes place in the highest temperature zone of the glass melting tanks. When attempts are made to melt the glass compositions containing above 80% by weight silica and a mixture of silica and alumina that comprises above 90% by weight of batch in special high temperature glass melting tanks, it has been observed that the temperatures obtained are high enough to just melt the batch and are not high enough to create the thermal currents necessary to intimately mix and obtain homogeneity of the constituents in the vitrified product.
In the high temperature glass melting tanks, the corrosion rate of refractories is extremely high, and the loss of fluxes for long periods at this high melting temperature is both undesirable and unacceptable. In high temperature melting tanks, the top temperature is restricted to slightly above 1600.degree. C. due to the restrictions on the capabilities of the firing systems and due to the limitations of the silica crowns in the melting tank. Attempts have been made to obtain high melting temperatures and reduce the above discussed problems. Electric melting has been utilized to obtain higher temperatures in the body of the melting mass while maintaining lower temperatures at the surface of the melting mass as well as at the refractory interfaces. The method of electric melting can generate temperatures in excess of 1700.degree. C. in the body of the melting mass while maintaining lower temperatures at the refractory interfaces.
In electric melting, the heat is generated by the ionic conduction taking place between two electrodes positioned in the glass batch with the glass batch acting as the electrolyte. The capacity of the glass batch to carry ionic current depends on the mobility of the various ions contained in the glass batch. In a high silica glass, the monovalent cations carry more than 90 percent of the current. Among the commonly occurring monovalent cations, i.e. Na.sup.+, K.sup.+ and Li.sup.+, the Na.sup.+ ions have a much high mobility than K.sup.+ ions. Therefore, in a glass composition where it is more desirable and preferred to have K.sup.+ ions rather than Na.sup.+ ions for reasons later discussed, it is difficult to attain the desired temperatures. For example; with a preferred composition that contains less than 3 percent by weight potassium oxide (K.sub.2 O), it is extremely difficult with electric melting to attain the high levels of currents and the prerequisite high temperatures required for attaining a suitable homogeneous melt.
In a conventional glass tank, the pulverulent constituents, commonly referred to as the glass batch, are fed to the tank through a suitable opening and are vitrified by melting. The melt, however, is not homogeneous in composition. To attain homogeneity, it is necessary to increase the temperature of the molten glass to provide thermal currents in the molten body. This results in a mixing of the molten material, and the composition thus becomes more homogeneous. Diffusion of the cations increases at the higher temperatures to also increase the homogeneity of the molten mass.
Removing the molten mass from the melting tank requires the molten mass to have certain characteristics, as for example, a sufficiently low viscosity to permit the glass to flow out of the tank. Again, these flow characteristics can be obtained by attaining sufficiently high temperatures in the melt. The temperatures required for obtaining homogeneity of the conventional glass compositions and for obtaining the required degree of fluidity are substantially the same, although the temperature for attaining homogeneity may be slightly higher. The level of temperature required to obtain homogeneity of the conventional glass compositions or for obtaining the necessary fluidity for conventional glass compositions is much lower than the temperature required to attain both the homogeneity and fluidity of glasses containing above 80 percent silica or having above 90 percent by weight of a combination of silica and alumina. In fact, the necessary high temperature to obtain both fluidity and homogeneity of the above high silica glass compositions cannot be commercially obtained in existing melting tanks now in common use.
In copending application, Ser. No. 685,054, entitled "A Pulverulent Borosilicate Composition And A Method Of Making A Cellular Borosilicate Body Therefrom", there is disclosed a process for preparing a cellular body from high silica borosilicate glass which includes preparing an aqueous slurry from an intimate mixture of colloidal silica, caustic potash, boric acid and alumina. The slurry is dried, and the aggregates are comminuted and thereafter calcined and rapidly quenched to form a ceramic frit. The ceramic frit is thereafter comminuted and mixed with a cellulating agent and introduced into a cellulating furnace and subjected to cellulating temperatures to form cellular bodies.
The high silica borosilicate cellular body formed according to the process set forth in the above copending application has the desirable properties of resisting degradation by an electrolytic salt bath and corrosive gases at elevated temperatures. The cellular body further retains physical integrity, especially insulating properties, under a load of about 17 p.s.i. at about 700.degree. C.
The above process now makes it possible to obtain cellular ceramic bodies which have the above desirable properties without the use of a melting tank. The process as described, however, requires calcining the entire batch and rapidly quenching the calcined material. As stated in the specification, it is preferred to use a plasma arc flame to calcine and rapidly quench the frit to prevent devitrification of the calcined material. There is a need to obtain a high silica borosilicate cellular body that has the above-discussed desirable properties without calcining the glass batch.
Attempts have also been made in the past to make cellular ceramic bodies from either naturally occurring glasses such as volcanic ash or from other materials that contain silica. For example, U.S. Pat. No. 2,466,001 and 3,174,870 disclose processes for making cellular products from volcanic ash, feldspar and granite. U.S. Pat. No. 3,441,396 discloses a process for making cellular materials from pulverulent materials that include fly ash. None of these processes however are directed to a process where a cellular body is formed from a high borosilicate composition such as a composition containing more than 80% by weight silica.